Recent years have seen development of a light emitting device obtained by combining (i) a light emitting element such as a light emitting diode (LED) with (ii) a wavelength conversion member that converts excitation light from the light emitting element into fluorescence (for example, a member containing phosphor particles dispersed in resin). The above light emitting device advantageously has a compact size and consumes less power than an incandescent lamp, and accordingly has been put into practical use as a light source for any of various image display apparatuses and illumination apparatuses.
Typically used one of such light emitting devices combines a blue LED and a yellow phosphor. As the yellow phosphor, Ce-activated yttrium-aluminum-garnet (YAG) phosphors have been widely used because of their high luminous efficiency.
In a case where a light emitting device is used in an image display apparatus, the image display apparatus has a wider color reproduction range at a smaller emission-spectrum half width of a phosphor. However, the Ce-activated YAG phosphor has a relatively-large emission-spectrum half width of approximately 100 nm. It follows that, in a case where the Ce-activated YAG phosphor is used as a yellow phosphor in a light emitting device and this light emitting device is used as a backlight device for liquid crystals of an image display apparatus, the image display apparatus will have a color reproduction range that is not sufficiently wide.
Specifically, the above image display apparatus can have a color gamut that covers substantially all the sRGB color gamut, which is used for cathode-ray tubes (CRTs). However, the image display apparatus has a considerably low coverage of the NTSC color gamut defined by the National Television System Committee (NTSC) or the Adobe RGB color gamut, each of which is wider than the sRGB color gamut and is used for wide-color gamut liquid crystal displays.
More specifically, in an image display apparatus which uses, as a backlight device for liquid crystals, a light emitting device including the Ce-activated YAG yellow phosphor, an NTSC color gamut coverage and an Adobe RGB color gamut coverage of a color gamut of the image display apparatus remain approximately 70% each. The above light emitting device is thus not suitable for use in wide-color gamut liquid crystal displays.
The “sRGB color gamut” as used herein means a color gamut represented by a triangle defined by three chromaticity points at (CIEx, CIEy)=(0.640, 0.330), (0.300, 0.600), and (0.150, 0.060) on the Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates.
Meanwhile, the “NTSC color gamut” as used herein means a color gamut represented by a triangle defined by three chromaticity points at (CIEx, CIEy)=(0.670, 0.330), (0.210, 0.710), and (0.140, 0.080) on the CIE 1931 chromaticity coordinates. Further, the “Adobe RGB color gamut” as used herein means a color gamut represented by a triangle defined by three chromaticity points at (CIEx, CIEy)=(0.640, 0.330), (0.210, 0.710), and (0.150, 0.060) on the CIE 1931 chromaticity coordinates. A comparison between the sRGB color gamut and each of the NTSC color gamut and the Adobe RGB color gamut shows that the NTSC color gamut and the Adobe RGB color gamut each have a green color reproduction range wider than that of the sRGB color gamut.
A light emitting device in which two phosphors, i.e., a green phosphor and a red phosphor, are used in combination is suitable for use as a backlight device in wide-color gamut liquid crystal displays whose color gamut corresponds to the NTSC color gamut or the Adobe RGB color gamut. Further, the above two phosphors each preferably have a small emission-spectrum half width.
For example, Patent Literature 1 discloses a light emitting device in which a Eu-activated β-SiAlON phosphor (green phosphor) and a Mn4+-activated fluoride complex (red phosphor) are used as phosphors in combination. When this combination is used to configure an image display apparatus, the image display apparatus can have a color reproduction range wider than that of a conventionally typical image display apparatus in which a yellow phosphor is used as a phosphor. This is because the Eu-activated β-SiAlON phosphor and a Mn4+-activated fluoride complex phosphor each have an emission-spectrum half width smaller than that of the Ce-activated YAG phosphor. Specifically, the Eu-activated β-SiAlON phosphor has an emission-spectrum half width of not more than 55 nm, and the Mn4+-activated fluoride complex phosphor has an emission-spectrum half width of not more than 10 nm.
Patent Literature 2 discloses an example of a light emitting device that can achieve a color reproduction range wider than that of the light emitting device disclosed in Patent Literature 1. This light emitting device makes use of a combination of a Mn-activated γ-AlON phosphor (green phosphor) and a Mn4+-activated fluoride complex (red phosphor) as phosphors. Patent Literature 2 also discloses that the green phosphor has an emission-spectrum peak wavelength of 510 nm to 550 nm and an emission-spectrum half width of not more than 55 nm (preferably not more than 45 nm). In addition, Patent Literature 2 discloses, as a production example of the green phosphor, a Mn-activated γ-AlON phosphor having an emission-spectrum peak wavelength of 515 nm and an emission-spectrum half width of 33 nm.
Patent Literature 3 discloses a light emitting device in which a Mn-activated oxide phosphor or a Mn-activated oxynitride phosphor is used as a green phosphor. Specifically, in the light emitting device disclosed in Patent Literature 3, the above green phosphor and a Eu-activated phosphor (red phosphor) are used in combination as phosphors. Further, Patent Literature 3 also discloses that the above green phosphor has an emission-spectrum half width of not more than 40 nm. As with Patent Literature 2, Patent Literature 3 discloses, as a production example of the green phosphor, a Mn-activated γ-AlON phosphor having an emission-spectrum peak wavelength of 515 nm and an emission-spectrum half width of 33 nm.
Patent Literature 4 discloses a production example of a color filter for use in image display apparatuses.
Patent Literatures 5 and 6 also disclose Mn-activated γ-AlON phosphors.